no SY, i saw that from a webpage many years ago by an english speaking guy....it does not use global negative feedback, the pentodes are run at less than 1ma current and i suspect that the gain is rather very high......

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the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....

But ultimate origin? Not RCA, I think. The input stage is a poor version of the Williamson (there must be almost 0.3mA going though the cathodyne!), the feedback arrangement is relatively ineffective, and RCA engineers were smart enough to understand feedback from the OPT secondary.

It is RCA. It appears in RC-19 receiving tube manual: circuit 19-11 on page 361. It is described as "High-Fidelity Audio Amplifier, Class AB1, Output 10 Watts". The verson linked to is slightly redrawn, but clearly the same circuit. No GNFB. Same component values. My guess is that RCA published it in different places, not clear with they would redraw it as printing plates were expensive.

My guess is that it is a low-power version of an originally higher power amp, where the extra driver stage might have a purpose. There is a 35W circuit, with some similarities, on the next page of the RCA manual. A 100-200W PA amplifier somewhere might be the original?